For some years it has been known that meaningful laboratory analysis can be performed using instruments which emit light of a specific wavelength onto a gas or fluid sample and measure the amount of that light which passes through the sample to derive an indication of absorption. The absorption may be indicative of the presence of an impurity in a liquid under test, of solute in a solvent, of the color of the liquid, of the presence of solid matter suspended in the liquid, or the like. Numerous instruments for such applications are known. The art has well documented the wavelengths of light which are absorbed by various materials so that the absorption of light of a specific wavelength is indicative of the presence of a particular material in the sample under test; if the amount of incident light and transmitted light are compared, an indication of the amount of the absorptive material may be derived.
Many of the instruments found in the prior art relate to measurement of the absolute amount of light of a specified wavelength passed through a sample when compared with a reference sample. Accordingly, many of the instruments in the prior art show splitting of a single beam into two identical portions, passing one beam through the sample under test and one through a reference sample, and comparing the amount of light transmitted so as to generate a result proportional to the amount of light absorbing material in the sample under test. Such instruments are very useful when it is desired to generate an absolute value for absorption. However, they tend to be very specialized--for detection of absorption of a particular element or compound--inasmuch as the reference specimen against which the test sample is compared must be changed if the instrument is to be used with differing materials. Moreover, it is rather unusual that various materials tested absorb the same wavelength of light, so that if the same instrument is to be used for testing differing materials, not only must the sample chamber be cleaned carefully and a new reference sample provided, but also the source of light must be modified to emit a different wavelength. While systems have been shown in the prior art which show means for emission of varying wavelength light, they are not as simple or as inexpensive as would be desired. For example, the SpectroMonitor III made by the assignee of the present invention uses a deuterium lamp to emit light of a wide range of wavelengths and then uses a diffraction grating to select the particular wavelength of light which is incident on the sample. This system is workable, but the diffraction grating is expensive and must be operated by precision mechanical means which add complexity to the system. In order to select the wavelength of interest, it would be desirable to manufacture an instrument having the capability of the SpectroMonitor III but at lesser expense. Moreover, the SpectroMonitor III is an instrument of the type described above, i.e., one in which the beam is split into equal portions for irradiation of a sample of known constituency and the sample under test. The beam splitter used is a fiber optic device which is expensive to manufacture and accordingly adds unduly to the cost of the instrument. It would likewise therefore be desirable to eliminate this optical device.
As noted above, the instruments in the prior art tend to be designed to generate a objective value for the quantitative difference between a reference sample and a sample to be tested. Such instruments are useful in laboratories where analysis of a given sample is to be performed. However, there is another class of application of spectrophotometric instruments which is not addressed by this type of instrument. This class of applications involves development of industrial processes. Such development typically will take place in a laboratory scale operation where a variety of parameters are to be varied to select the optimum operating conditions for a given process. For example, there is at present a publicly felt desire for decaffeinated coffee wherein the caffeine is removed from the coffee using non-poisonous or non-carcinogenic solvents such as carbon dioxide. Processes are being developed all over the world for the removal of caffeine from coffee beans using supercritical carbon dioxide, i.e., carbon dioxide raised to a temperature and a pressure above its critical point, so that the gas behaves as a solvent. There are numerous variables in the supercritical solvent extraction process and it would be desirable to be able to vary them on a more or less continuous basis and to provide a continuous indication of the efficacy of the process at any given time. Accordingly, it would be desirable to provide an instrument which would provide a substantially instantaneous indication of the amount of caffeine in the supercritical CO.sub.2 stream leaving the apparatus, so that an experimenter could make an adjustment to the process parameters and immediately see whether or not the efficiency of extraction had been improved. It will be recognized by those skilled in the art that such an instrument need not provide an absolute value of the amount of caffeine in the supercritical CO.sub.2 stream, but merely provide an accurate indication of whether or not the amount had increased or decreased since the process parameter was changed.
As discussed above, most of the instruments in the prior art for spectrophotometric measurement have split the beam of incident light of given radiation into two equal parts so as to equally irradiate a sample to be tested and a reference sample. This is done in order to insure accuracy of the objective measurement provided by the photocell exposed to the radiation passing through the sample to be tested. However, splitting the beam into two parts means that the energy output by the lamp is split as well, which is undesirable in the case of darker, more absorptive samples as then more amplification is necessary to turn the lower photocell output into a meaningful output signal. It will be appreciated by those skilled in the art that as the photocell output signal drops, it gets more and more closely related in amplitude to the noise inherent in any electrical system. Accordingly, it would be desirable, for a given lamp, to direct as much of its beam as possible on the sample so as to generate a larger output signal, thus raising the effective signal-to-noise ratio of the system.
Another deficiency of prior art spectrophotometric measurements when applied to modern processes now under development such as supercritical carbon dioxide extraction of caffeine from coffee beans is that these processes typically take place under very high pressures, e.g., 10,000 psi. If an instrument is to be interposed "on line" in the system, it must be capable of withstanding such pressures. No instrument is known to the inventor which satisfies this requirement. Instead, previous spectrophotometric instruments require that a sample be taken and transferred to the instrument at a lower pressure, which greatly complicates the process and renders an on-line instrument unfeasible. This, in turn, renders easy optimization of the process parameters by obtaining an instantaneous read-out of the effective variation of a process parameter impossible.